Mice
Mice are generally much smaller than rats. Proportionally they have smaller ears and smaller hind legs. Mice can squeeze through holes slightly larger than a pencil eraser. They commonly slip in through open doors, attics or foundation vents.
Rat
Both the Roof rat and Norway rat are the most common rodents in the United States. Rats are larger than mice and have proportionally smaller ears and bigger hind feet. Juvenile rats are distinguished from adult mice by their larger feet, head, and tails. They can gnaw through furniture and electrical wires, and carry infectious diseases.
